Certain advantages may be realized by operating the steam turbines of electrical power generating stations with constant or sliding pressure boilers. This mode of operation permits the steam boiler to be maintained at a high steam production rate independently of the load demand on the steam driven turbine and is attained by using a bypass arrangement to divert the excess steam around the turbine directly to the condenser during periods of low turbine loading. As load on the turbine is increased, more steam flow can be apportioned to it and less bypassed until a point is reached at which all of the steam is devoted to the turbine and none bypassed. Once the bypass is completely shut off the boiler pressure may be allowed to increase, or slide upward, to its rated pressure in support of the turbine demand for steam. Conversely, with a lessening of turbine load, the boiler pressure may be allowed to slide down to some acceptable minimum level, followed, if necessary, by again bypassing the excess steam. Among the principal advantages of this kind of operation are (1) shorter turbine startup times; (2) use of larger turbines for cycling duty where there must be a quick response to changes in load; and (3) avoidance of boiler trip-out with sudden loss of load. A general discussion of the sliding pressure mode of operation appears in Vol. 35, Proceedings of the American Power Conference, "Bypass Stations for Better Coordination Between Steam Turbine and Steam Generator Operation", by Peter Martin and Ludwig Holly.
Contrasted with the more conventional mode of turbine operation (wherein the boiler generates only enough steam for immediate use and where there are no bypass valves), the sliding pressure mode necessitates unified control of a more complex valving arrangement. The control system must provide precise coordination of the various valves in the steam flow paths and do so under all operating conditions while maintaining appropriate load and speed control. There are three principal phases to consider in the operation.
1. With the turbine down and the boiler at reduced pressure, or being started up, steam must be bypassed from the main steam line to the cold reheat line, and from the hot reheat line to the condenser by means of pressure-controlled bypass valves;
2. Upon turbine startup, the control and intercept valves should open according to a relationship that maintains reheat pressure at a predetermined level regardless of main steam pressure and in coordination with the bypass valves for unified control of the boiler and reheater pressures; and,
3. At a predetermined turbine load the bypass valves should become fully closed, the control valves held in approximately constant position, and the boiler pressure ramped up to rated pressure by increasing steam flow.
Various control systems have been developed for reheat steam turbines operating in a sliding pressure regime. In one known scheme, pressure in the first stage of the turbine is used as an indicator signal of steam flow from which reference setpoints are generated for control of the high-pressure and low-pressure bypass valves. There are no provisions, however, for directly coordinating the bypass valves with operation of the main control valve, which must be responsive to speed and load requirements, nor for coordination with other valves of the system. Furthermore, it is recognized that first stage pressure is not a valid indicator of steam flow under all prevailing conditions.
In another known sliding pressure control system, a flow measuring orifice in the main steam line provides a signal indicative of total steam flow, forming the basis for a pressure reference signal for control of the high-pressure and low-pressure bypass valves. The flow measurement thus made requires an intrusion into the steam flow path, a corresponding pressure drop, and additional equipment not normally available.
The fundamental signals upon which these and other prior art systems depend for control are derived from sources other than the controller responsible for maintaining turbine speed and load. Thus, in these previous systems there has been a group of somewhat independent control loops; one for speed and load, others for the bypass valves. An object of the present invention, therefore, is to provide a comprehensive control system for turbines in the sliding or constant pressure mode of operation wherein the speed and load control means is incorporated into a unified system for control of all valves, and wherein operation is coordinated with control of boiler and reheat pressures by automatically positioning the main control valve, the intercept valve, and the high- and low-pressure bypass valves.
Another object of the invention is to provide an improved and unified control system for reheat steam turbines operable in conjunction with sliding or constant pressure boilers and wherein automatic control is effective during all phases of turbine operation.